<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
</head>
<body>
  <button id="btn"></button>
  <script>
     //Map
     //key value
     const btn = document.getElementById('btn');
     var map={
       name:'lihua',
       [btn]:'dom node',
     }
     map['4a'] ='a val';
     map['3b'] ='b val';
     map['2c'] ='c val';
     map['0d'] ='d val';
     let map1= new Map([
       ['key','value']
     ])
     console.log(Object.keys(map));
     console.log(map1);
     //有什么用
     //为什么 es6 要增加一个Map
     //1.以前的key 只支持字符串
     //2.以前是无序的 现在是有序的
    </script>
</body>
</html>